1.Technical Field of the Invention
The present invention relates generally to a fuel injection system which may be employed with automotive internal combustion engines to learn a deviation of the quantity of fuel actually sprayed by a fuel injector from a target quantity to produce a correction value for correcting an injection duration for which the fuel injector is to be opened to spray the fuel desirably.
2.Background Art
There are known fuel injection systems for diesel engines which are designed to spray a small quantity of fuel into the engine (usually called a pilot injection) prior to a main injection of fuel in order to reduce combustion noise or NOx emissions. However, a deviation of the quantity of fuel actually sprayed from a fuel injector from a target quantity in the pilot injection will result in a decrease in beneficial effects of the pilot injection.
In order to avoid the above problem, Japanese Patent First Publication No. 2005-155360 proposes a learning control system which is activated when the diesel engine is decelerating, and no fuel is being sprayed into the diesel engine. Specifically, the learning control system instructs a fuel injector to spray a single jet of a target quantity of fuel into the diesel engine, samples a resulting change in speed of the engine to calculate the quantity of fuel actually sprayed from the fuel injector, and determines a correction value for an injection duration for which the fuel injector is to spray the fuel (i.e., an on-duration for which the fuel injector is opened) based on a difference between the target quantity and the actually sprayed quantity of the fuel (which will also be referred to as an actual injection quantity below).
The learning control system works to use the correction value to regulate the injection duration so as to bring the actual injection quantity into agreement with the target quantity. The learning control system is designed to determine the injection duration in the learning mode directly as a function of the target quantity, thus giving rise to the problem that the correction value may have an error due to an injection characteristic (i.e., a relation between the injection duration and the actual injection quantity) unique to the fuel injector.
For example, the injection characteristic representing the relation between the on-duration Tq (i.e., the injection duration) for which the fuel injector is to be energized to spray the fuel) and the quantity of fuel sprayed from the fuel injector (i.e., the actual injection quantity) is, as demonstrated in FIG. 5, usually different between fuel injectors A, B, and C. The injection characteristics of the fuel injectors B and C are different in inclination thereof greatly from a basic injection characteristic which is designer-predefined.
The learning control system works to energize the fuel injectors for a constant basic on-duration Tqo, as selected from the basic injection characteristic, to spray the single jet of fuel and calculate a deviation of a resulting quantity of fuel actually sprayed from the fuel injector and the target quantity, and determine the correction value for the basic on-duration Tqo using such a deviation and the inclination of the basic injection characteristic.
Consequently, in the case where the fuel injector A whose injection characteristic is identical in the inclination with the basic injection characteristic is required to be learned, the correction value is calculated correctly to bring the actual injection quantity into agreement with the target quantity, while in the case where the injector B or C whose injection characteristic is greatly different in the inclination with the basic injection characteristic is required to be learned, it will result in a decrease in accuracy in determining the correction value, which leads to an error in bringing the actual injection quantity into agreement with the target quantity.
The above problem may be alleviated by instructing the fuel injector to spray the single jet of fuel several times cyclically for different injection durations, sampling combinations of the actual injection quantities and the injection durations to calculate the injection characteristic (i.e., the Q-Tq characteristic demonstrated in FIG. 5), and determining the correction value to correct the injection duration for which the fuel injector is to be opened to spray the target quantity based on the calculated injection characteristic.
The determination of the correction value with increased accuracy in the above manner requires even dispersion of the injection durations (i.e., the actual injection quantities) around the target quantity in a sequence of the injections of fuel into the engine through the fuel injector.